This invention relates to a method for removing acid or saline contaminants from a gas-vapour mixture leaving a dimethylcarbonate synthesis reactor.
Dimethylcarbonate (hereinafter known as DMC) is a widely used chemical product of great versatility, employed as such as a solvent or as a fuel additive; DMC is also an important intermediate in the synthesis of alkyl or aryl carbonates, which are used as synthetic lubricants, solvents, monomers for polymeric materials and for the preparation of isocyanates, urethanes, ureas and polycarbonates.
The path currently most followed for producing DMC is based on oxidative carbonylation of methanol, in particular in the presence of CuCl as catalyst, in accordance with the reaction: EQU 2CH.sub.3 OH+CO+1/2O.sub.2 .fwdarw.(CH.sub.3 O).sub.2 CO+H.sub.2 O
The preparation of DMC in accordance with this reaction is described for example in U.S. Pat. Nos. 4,218,391 and 4,318,862 in the name of the present applicant.
Improvements to the processes described in said U.S. patents are introduced in European patent applications EP-A-460,732 and EP-A-460,735 in the name of the present applicant, the content of which forms part of the present application as reference. These applications describe a continuous DMC synthesis process in which the reaction products are removed from the reactor in the vapour phase. Leaving the reactor in this process there is a saturated gaseous stream containing vapour of the water/methanol/DMC system plus unreacted CO and O.sub.2, CO.sub.2 deriving from a second reaction and possibly inert gases present in the feed to the reactor (H.sub.2, Ar, N.sub.2 etc.). This gas-vapour mixture is passed through a condenser which separates a water/methanol/DMC liquid mixture from the uncondensable gases, which are largely recycled to the reaction. The water/methanol/DMC liquid stream is then fed to the separation section which by distillation and liquid-liquid separation recovers the DMC and the water produced, and recycles the unreacted methanol to synthesis.
This process has however the drawback that the gaseous stream leaving the reactor is contaminated with a small quantity of hydrochloric acid of generally between 30 and 300 ppm by volume, which is released from the catalyst used in the reaction. In addition to HCl, the gaseous stream leaving the reactor can also contain small quantities of halogenated copper salts deriving from catalyst entrainment in the form of particulate and/or droplets of micron dimensions. The amount of copper transferred in this manner is generally between about 1 and 20 mg Cu/Nm.sup.3.
The presence of chloride ions and possible copper ions results in considerable problems in those plant sections downstream of the reactor. The stage in which the gaseous mixture leaving the reactor is condensed is particularly critical. In this respect, it is not possible to condense this gaseous mixture in a conventional steel condenser because of resultant serious equipment corrosion by the HCl. Again, the use of corrosion resistant materials such as enamelled steel is technically unfit because of the fact that these materials generally have a low heat transfer coefficient, so that a condenser of prohibitive dimensions would be required for condensing a large acid gas throughput, taking into account the high price of such materials. Operating under conditions of complete condensation using corrosion resistant plants could represent an acceptable solution to the aforesaid problems if operating in low production plants with equipment of small dimensions. However even in this case there would still be the drawback of having to process the entire condensed liquid mass, which before being fed to the product recovery section would have to be neutralized and finally separated from the salts formed by neutralizing the acid.